The invention relates to a process for the permanent joining of a cylindrical core of a heat-insulating material to a beaker-shaped part of metal, which together form the operating piston of a hydraulically operated disc brake, wherein the core is inserted into the beaker-shaped part and the beaker-shaped part is then plastically deformed in the area of its annular surface, whereby metal is displaced inwardly so that it extends over the core.
It is the object of two-part operating pistons of this type to prevent the leakage of heat, which is produced during the braking by way of friction, to the hydraulic medium acting on the pistons. For safety reasons, the temperature of the hydraulic medium must never exceed 120.degree. C., otherwise vapor bubbles develop and substantially restrict the functional capability of the brake.
Various types of joinings of a core of heat-insulating material to a metal beaker-shaped part of a brake piston can be divided into three main groups:
1: Permanent joining PA1 2: Releasable joining PA1 Disadvantages of 1 and 2 are: PA1 complicated piston geometry; PA1 small individual parts, placement, and manufacturing tolerances; PA1 many individual parts; PA1 large stock of spare parts; PA1 complicated and expensive assembly, dismantling and repair. PA1 3: Loose joining PA1 complicated piston geometry; PA1 small placement and manufacturing tolerances; PA1 different wear parameters (metal/synthetic material).
Adhesive joining can also be considered for a permanent joint, however, present day technical possibilities include the danger of separation due to different heat-expansion coefficients and with the development of dynamic loads. Furthermore, the technical effort for producing an adhesive joint is large.
Disadvantages of 3 are:
Joinings according to 2 and 3 may also result in destruction due to different piston materials if dynamic loads develop during a braking process. This can occur either by disc warpage or by brake application when brake forces, which are effective in the peripheral direction of the brake disc, force the shell surface of the cylindrical core against the abutment surface of the beaker-shaped metal part.
A method of the earlier described type is described in European Patent Document EP-0 042 156 A1. No statement is made in this document regarding the fitting of the cylindrical core and its receiving bore in the beaker-shaped part. After inserting the core into the beaker-shaped part, its annular surface protrudes axially over the end surface of the core. The protruding area of the beaker-shaped part is then flanged radially inwardly, thus forming a lip which reaches over the face surface of the core. An additional metal disc, which abuts against the face surface of the core and is permanently connected thereto, is provided in order to avoid contact of the flanged lip with the brake shoe. For this reason, the metal disc is provided at its rear side with a hollow hub, which is inserted into a concentric bore in the core of a heat-insulating material and then radially spread in its protruding area. Obviously, the manufacture of this known brake piston is relatively expensive. Consequently, necessary repairwork is rather expensive if the complete brake piston has to be exchanged.